High-frequency tube structure



Oct. 21, 1947. D. L. sNow ErAx. 2,429,243

HlGrH-FREQUENCY TUBE STRUCTURE l Filed June 18, 1942 5 Sheets-Sheet l l :l\\\\`l\\2 hill. 42O INVENTORS: D.'1 .sNow E. L.. omz-roN BY D. R. HAMILTON 'ATToRNEf Oct. 2,1, 1947. D. l.. SNOWA ET AL. 2,429,243

Y l HIGH-FREQUENCY 'IIUBE STRUCTURE File'dJune 18, 1942 s sheets-sheet 2 sa' 5g ATTORNEY Oct. 21, 1947. D. l.. sNow- Er AL HIGH-FREQUENCY TUBEI STRUCTURE Filed June 18, 1942' s sheets-sheet s FIG. 9A

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, ...mwmwhl ATTORNEY ported in a focusing cylinder 4 on which is mounted a mesh wire grid 3 directly in front of emitter I, both cylinder 4 and the hot cathode being supported from insulator 5. Lead wires 2 feed current to a heater coil (not shown) inside of cathode I. Voltages are applied to leads 2 and to focusing shield i by means of leads 5, 1, respectively, which pass through the glass portion 8 of the vacuum envelope. The cathode structure is centrally positioned in a cylindrically apertured conducting member 9, which also supports resonator entrance grid I parallel to cathode emitter surface I. Grid I0 may consist of alternate short and long conducting radial grid wires. Member 9, together with flexible conducting diaphragm I2 mounted thereon, forms one wall of a cavity resonator, the remainder of the resonator being enclosed by resonator body II.

As shown in Fig. 2 of the aforementioned U. S. Patent No. 2,250,511, there is usually provided op posite the entrance grid IU and positioned centrally in resonator body II, a second or exit grid similar to grid I0, through which the electrons from cathode I pass. After the electron beam leaves the resonator it is reflected back thereto by means of `a reflector electrode operating at or near the potential of the cathode. If an ultra high frequency electromagnetic eld existswithin the resonator, the electron beam undergoes recurrent velocity changes by the action of the field during the beams initial passage through the resonator. As the electron beam passes beyond the resonator toward the reflector electrode, the effect of the velocity modulation is to commence recurrent grouping of the electron beam. The spacing between the resonator exit grid and the reflector electrode, and the voltage on said electrode are so arranged that the electrons are reiiected back into the resonator at a time when the velocity modulation has resulted in density modulation producing electron groups and the electron gro-ups return energy to the electromagnetic field thereby maintaining it. Direct current, introduced into the device by means of the electron beam, is thus converted to ultra high f requency energy and may be removed from the resonator by means of coupling loops and associated concentric line elements.

In the present embodiment of the invention, however, the exit grid is dispensed with, and the reflector plate is introduced in a novel and useful manner. As seen in Fig. 1, the reflector surface I8 is positioned substantially flush with the inner conducting surface of resonator body II. The surface I8 is the fiat end surface of a quarterwave long cylinder I9, which forms a portion of the iirst section of a low pass iilter consisting of an outer cylindrical conducting tube I1 surrounding concentrically an inner conducting member comprising approximately quarter-wave long sections I9, 20, 2I, 22 which are alternately large and small in diameter. The last small diameter section 22 of the iilter protrudes through glass to metal seal 23, thus providing support for the inner conductor of the lter. The lter is provided in order that a voltage equal to the cathode voltage, or some other desirable voltage may be supplied to reflector surface I8 without the loss of ultra high frequency energy from the oscillating electromagnetic iield inside the resonator. Any number of iilter sections may be employed; however,

four alternately large and small diameter sec-1 tions ordinarily `provide adequate attenuation. The design and operation of such lters have been described in copending application Serial `No.A

457,095, entitled High frequency filter structure, filed September 2, 1942, in the name of William W. Hansen.

Energy may be removed from the resonator by coupling loop 24, which is shown at the termination of coaxial line 25. In very short wave devices of this type, the resonators may be so small that coaxial lines of conventional size cannot conveniently be utilized; consequently line 25 is made very small in diameter. Both conductors are expanded in tapered section 26 to adapt the line to a section 21 of normal size. Continuity of the vacuum envelope is provided in line 21 by the glass-to-metal seal 28. Tubular outer conductor 29, with its threaded flanged portion I4, serves in the conventional manner as a connector to a conventional coaxial line to lead energy to utilization apparatus. Additional coupling loops may be supplied, if desired.

' A spider, fixed on reentrant member 9 and having three legs I3 spaced opposite three legs I5 attached to resonator body II, forms, together with screws I6, means for tuning the resonator by variation of the space between grid I0 and reiiector I 8, in the well known manner. Other well known tuning devices, as shown in later figures, may be used if desired.

One mode of operation of the device may be described with reference to Fig. 2, illustrating an idealized graph of space potential as a function of distance between the cathode I and the reflecting surface I8. The reflector is assumed to be at cathode potential, and the eiects oi space charge are neglected. As seen in Fig. 2, the potential on any electron in the space between cathode I and reflector surface I8 is an increasing function of the distance from cathode I. After passing through grid ID, the electron is acted upon by a eld which is the sum of a constantly decreasing potential and the alternating potential due to the ultra rhigh frequency oscillating field between grid I0 and reflector I8. Dotted lines 48, 48 represent extreme conditions of this resultant field. Electrons, accelerated by the direct Voltage between emitter I and grid I0, pass through grid I0 with a uniform velocity, and while traversing the space from grid I0 to their point of reection and back through the grid I il, are velocity modulated in the conventional manner. After passing through grid I0 in the reverse direction, the electrons are reflected in front of the cathode structure and are again directed through grid I0. During this last reflection, the electron beam has become density modulated, and returns to the space between grid I8 and reflector I8 to give up energy to the oscillatng electric eld therebetween, thus maintaining electromagnetic oscillations in the resonator. It is seen that the distance between grid I and the point of reversal of the paths of the electrons in front of reflector I8 and cathode I is determined by the flight time necessary to bring the density modulated beam into the oscillating electric iield at the proper instant to give up. energy to that eld.

Analysis of another mode of operation of the device shown in Fig. l may also be made similar to the analysis of operation of the electron beam velocity modulating reiieX devices disclosed in the above mentioned U. S. Patent #2,250,511. Electrons from emitter i are focused into a slightly converging circular cross-section beam by focusing shield 4, and have been accelerated by the cathode voltage on reaching entrance grid i0. in traveling from grid I0 toward reector surface I8, the beam is velocity modulated. With proper interelectrodef-spacings, and with the :proper reiiector voltage, .all of :the electrons are reflected before :reaching surface lf3 .and travel again toward lgri-dsll. During .ther-interval of reflection, thevelocityimodulation of the electron Aloearn has resulted indensitymodulation, in such a'manner that theseelectrons .give up energy to the-oscillating-field thus-maintaining it. It is againseenthat inter-electrode `distances Vare determined by the necessaryelectron transit time.

If the voltage impressed on reector surface I3 is not sufcient to -reiiect those electrons which are most accelerated during the velocity modulation process, they will-be collecte-d bythe reiiector. Surface I3 may be provided under such circumstances with a secondary electron suppressing material. The action lmay'ber-spoken vof as .selectron beam `density modulation yby absorption Suppose that all electrons passing grid l0 during thehalf ofthe cycle in-Which'the oscillating iield aids-the beam acceleration voltage strilethe reflector `I8 4and are collected. These electrons remove energy from'the field. Electrons entering theresonator one-.half `cycle later find the oscillating fieldy opposingtheinmotion, and thus supply energy to the field. These electronsare so deceleratedzbythe retardingpotential of reflector I8 .that-they are reversed in ydirection'without striking the reflector. As the reflected electrons travel 'back towardgrid lil, the oscillating eld has again reversed; Vconsequently the electron-s again addenergy to .thefleld Theresult is a positive addition-of energy to the oscillating electromagnetic eld, .so that theeld is maintained. The device of Fig. l, as wellas alternate forms to` be iurthendiscussed in .connection with later figures may thus operate assimple velocity modulation device, oras an absorption density Imodulation device, asdesired.

Fig. 3 villustratesa slight modification inthe configuration Yof the electrodes of the device of Fig. 1. vThe cathode and heat or .focusing shield 4 are seen to be similar to those of Fig. l. The radialgrids I@ of Fig. l are, however, replaced by aknitted or woven grid, which is preferably made of aconducting material or may be plated witha conducting material. The grid 3@ is made to'bow away'from the focusing shield 'il into the resonator space. The iirst section lii^ of the lter is modifledso that its upper reilecting suiface I8' is concave. The surface i8 may be parallelto the surface 3B or may not, as desired. The actual con-figurationis empirically adjusted vto -give best focusing of the elect-ron beam, and lit seems evident that one skilled in the art'may use many modifications of the grid and reflector kplate shapes -shown-in Figs. -1 and v1%.

If the'device of Fig.'1 is operated'bythe-absorption fbun'ching'pr-inciple, the modification shown in -F-lg. f4 is useful. In this figure the first-:section I9 of the filter-is modified so that the -refleeting surface I8 of -Fig. l is replaced by a grid 33iwhich lmay be a Woven or knitted grid. `The cylinder I is made hollow and contains-inits bottom a reentrant conical projection-34; In operation, the electrons which pass the grid l0 at atime whenfthe oscillating field in the resonator accelerates'them, pass through the grid 33 and are collected by the Faraday cage action of the element i9. The `cone 3d is provided so that secondary electrons emitted when high energy electrons strike its surface will not be reflected back'into the resonator cavity. Alternatively, if desired, the surface I8 of the reector may be coated with a secondary emission substance, so

6 that',densitymodulationfis causedfwhenlfilgh-zspeed velocityfmodulatedelectronsistrik'e the.;surface -I 8:

Vlith reference totFig. 1:.it twas stated thatifor Very"small:wavelengthzdevices `:of 'thisrtype iritxmay sometimes be f diiiicult :to :introduce rcouplinggloop 24 anditszassociated coaxial line.. 1Fig..;5 showsa modification of 'the .structure ofFig. 1 combining the energy-removing "device fand theffilter through'xwhich the .reector voltageiis introduced. Only section 5'I'9 vof :the'rlter '.is v:used, concentri-v callyresupportedin arr .outer conducting .tube "35 by an finner conducting.trod ill'ii. Rod A6 4:is :in .turn supportediby iaf'rod3.61whichpasses :out through a glassf-.tofmetal seal '38 dnfth'ezend :'ofitubef35i. .At the junction 4.between A.rods 36 and lid-rod 53.6 Jfhas a small diameter-.projection -41 twhich'. is a. quarter wave-.long and which' is inserted. .into :a central hole Yin rod 4B, rods '36 and `46 being insulated from eachother by ymeans of `an insulating;A bushing 'lilil approximatelya quarter wave long. At tachedzat right-anglesito rod 46 is a small diam.- eter innerconductor '40 which passes through Aa hole iin..outertube 35. Outside of vtube 35,-.conductor '40 :is concentrically surrounded :by 1a. aconducting` tubef39.- 'Near the outer end of tube/239 the inner conductorll' is vexpanded into .aa-section- 43..;a1most 'equal the inner diametero'fthe tube :39 and vinsulatedctherefrom by an insulating-*bushing 44. -Section 1:43. of the .line'is made approximately f a :quarter kwave long and i is terminated by:a lead l45.- A'iglass :.to'metalseal A2 iis provided to give :continuity 'tothe vvacuum .envelope.

VInthe-operation y.of Fig. f5, a voltage vequal .fto or nearccathode voltage is applied tothe llead i5-andV is thus directly introduced tothe reflector plate-surface I8. The `filter section I9 -allows an-arbitrarily chosen amount of energy to flow downfthe concentric line '46, .35 and may be designedso that amaximum amount of ultrahigh frequency energyfcanlberemoved from `the-resonator without :overloadingit and causing oscillations :to stop. .The .insulating section -4B rprevents the reflector voltage from going out onthe inner .conductor "36 "to utilization apparatus, fand thefllterrsection 43 prevents ultra high frequency fromzxleakingout into space, the coaxial line 39,-540 being 'designed to have very high impedance-at the frequency used. The-tube35fis providedfwith a-zthreaded flanged portion 31, to'which-:a confventional coaxial cable may be attached. Filter section I9 may be a vhollow Faraday cage similarto section I9" of Fig. 4,'if desired.

. Referring now to Fig. 6, there is illustrated a modification of the device of Fig. 1 vin whichv thermal tuning of the resonator isreadilyaccomplished. The cathode structure may be similar to that previously described. A hollow'resonator 55 is formed by a centrally apertured conducting plate 56, a cylindrical conducting tube 5l, and conducting `member'ii with a protruding portion 54. Portion 54 has a surface 53 that may be utilized as a reflecting electrode. Tube 5l 'is extended to provide the outer conductor'of a coaXial'flter whose inner 4conductor comprises alternately large and small portions 53, 59,165,

and 6I. A centrally apertured conducting plate 63 closes the tube 51 and terminates the lter. The last small diameter portion EI ofthe lter consists of a thin walled conducting'tube'which' has an extension 65 projecting through the aperture in the plate 63. Extension 65 is concentrically surrounded :by a-tube E4 attached to the plate B3. A glass seal'BS between tubes lillV and 65 provides continuity to the Avacuum envelope as Wellas an insulating support for the inner n1- ter structure. This construction allows .the 'surface 53 to be maintained at a desired potential with respect to the entrance grid l while preventing radiation loss from the resonator 55 through the glass seal 66. Tube extension 65 contains a cartridge or other type of heater 52 to which power is applied by leads 68 passing through an insulator 61. Power applied to the heater B2 causes expansion of the thin walled tube 6|, 65; consequently the spacing between' the entrance grid IIJ and the reflecting surface 53 is altered by the motion transmitted down the entire inner conductor of the filter. Thus the resonant frequency of the resonator 55 may be varied at will in the well known manner.

Electrical Vtuning may be readily effected in the device of Fig. 6 by variation of the voltage applied'to the reflector electrode 53. As is well known in the art, such a tuning method is most useful with low Q, high shunt impedance resonators. If the protruding portion 54 (and the electron beam) is made very small in diameter as compared'to the diameter of the resonator, a low Q resonator results. If desired, these features maybe combined in the device of Fig, 6, electrical tuning being used to provide changes over small frequency ranges, and thermal tuning being employedV to provide changes' over large ranges. If desired, the protruding portion 54 of the resonator 55 may be omitted, as seen in Fig. 7. In this case resonator 55 has a very high Q and has a considerably lower effective shunt impedance than the resonator 55 of Fig.- 6.

Referring now to Fig, 8 there is illustrated a form of the invention in which the entrance grid to the resonator is omitted, an exit grid being supplied. vAn emitter surface 8| of a cathode 89 is placed Vcoincident with the usual position of the entrance grid and a lter is provided around the cathode 8l).V The cathode 86 has a long tubular heat shield mounted on insulating washer 82 and is-supplied with heating power by leads 83 connecting to leads 86 which pass out of the vacuum envelope through glass to metal seals 81, Concentrically surrounding the cathode Eil are small and large diameter conducting cylindrical tube sections 89, 9|), 9|, 92, and 93 respectively. Annular plates 94, 95, 96, and 91 connect adjacent dissimilar diameter tube sections Yto provide electrical and structural con'- tinuity. The tube 8|! together with the composite outer conductor form a low pass filter which functions similarly to the filter of Fig, 1. YIt is to be understood that the two forms oi the iilter. .may be interchangeably employed according to mechanical convenience. A lead 85, passing through the glass seal 81 and connecting to a lead 85, is provided to maintain the cathode at the desired negative potential. rihe last portion 93 of the ltery is fastened to an annular lplate 93 which comprises, together with cylindrical tube |33 and opposed apertured plate lul, the resonator |08. A cylindrical tube |13 supporting an exit grid |62 projects through the aperture in plate lill. A conventional reecting electrode |118 mounted on a conductorV |09 is held within the tube H0 in the proximity of the grid |82 by a glass seal which both provides continuity to the vacuum envelope and, electrical insulation for the conductor Idil.A Tuning of the resonator |09 is shown to be accomplishedby a conducting or dielectric plug H3, which, by rotation of knob I I5, may be inserted to a greater or lesser de grec; into the resonator. Reentrant glass tube ||4 surrounding the plug ||3 is sealed to a con? centric metal tube ||2 inserted in the side |03 of thevresonator to provide continuity of the vacuum envelope. The operation of such a tuning device is to cause distortion of the electromagnetic field inside of the resonator, thus altering its natural frequency in a well known manner.

Fig. 9 shows a form of the present invention most useful when operated as a simple electron beam reflex velocity modulating device. A cathode 3ii is seen mounted in a concentric tubular focusing shield |32 having at its outer end a focusing grid [3| which is substantially parallel to the emitting surface of cathode |30. An entrance grid |31 to a resonator |38 is made substantially conical, the cone pointing toward the center of the grid |3|. rI'he exit grid of the resonator is again omitted, the reflecting voltage being applied through a filter |43 to a conical protruding reflector|45 whose surface may be parallel to that of grid |31. Electrons from which energy has been extracted are now reflected toward a grounded annular plate |33 surrounding the focusing grid |3|, so that substantially none of these electrons are allowed to strike the emitting surface of the cathode.

Fig. 10 shows a slight modication of the device of Fig.`9 which is useful for a similar purpose. A cathode |10 is mounted with its axis of symmetry at an angle with the axis of symmetry of a resonator |14. The electron beam of cathode |10 is projected through a large opening in a wall |16 of the resonator |14 containing radial grid bars |11 and is again reected by a reector surface |19 supported by a lter |8| similar to that of Fig, l. Spent electrons are allowed to pass through the grid |11 and are collected by conducting wall |15. It seems apparent that any dedesired method of tuning of the resonators of Figs. 9 and 10 may be used. The subject matter of vliigs. 6-10 is described and claimed in our copending divisional application Serial No. 734,690.

As many changes could be made in the above construction and many apparently widely different embodiments of this invention could be made without departing from the scope thereof, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

l. High frequency tube structure comprising means for producing an electron stream, a hollow resonator having electron permeable entrance means in the path of said stream and an opposite apertured wall, a cylindrical insulated reiiector electrode positioned within said aperture and having its end substantially ush with said wall, and a tubular member surrounding said reflector, said reilector being substantially a quarter-wavelength long at the operating frequency of said structure, whereby it forms a lter with said member for preventing leakage of energy past [said renector.

42. A thermionic tube structure comprising a hollow resonator having opposed apertured walls, anrentrance grid occupying an aperture in one of said walls, a reecting electrode occupying an aperture in the opposed wall, means aligned with said grid and electrode for projecting a beam of electrons through said entrance grid in a manner to be reiiected by said electrode for exciting electromagnetic waves Within said resonator, and means including said reecting electrode as an element thereof for 'abstracting 4aJ predeterminable amount ofenergy from2said resonatom 3. vAn ultra high frequency oscillator comprising means for-*emitting*electrons; means forform'- ing theemitted electrons-intoa beam,` anlelectro'- magnetic resonator adaptedl torcontainw an oscillating electric-l fl'eldtherein-in thet path of' said beam, whereby-velocity modulationof' thebeam is producedby the oscillating-'electric' field-contained within` said resonator; a conducting'- electrodesubstantially ushi-'with alwall" ofi said resonator and' Ainsulated therefrom, 'means connectedbetween said` resonator andsaid electrode for maintaining af' potential -dil'erencebetween saidf resonator and said: electrode ter-oppose the electron beam and eife'ct"density'moclulation thereof, whereby the-density modulated beam delivers energy to theoso'illating fieldto4 maintain the same, filter means` including` said electrode as an element thereof toprevent radiation* from said'resonator, and lmeans coupled tov saidresonator for-'a-bstractingenergy therefrom.

4. A-generator-'of electromagnetic waves; comprising an electromagnetic resonator'adapted to contain an oscillating-electric` field thereinV and having'rst and second'electron permeable walls, means alignedwith said resonator for producing an electron beam and for projecting' said electron beam throughroneofsaid permeable walls inton said `resonator tof-effect velocity modulation of the-beam'lby the field therein, means including said -second electron p ermeablewallfor reecting electrons of thebeamrwhereby energy is supplied to: the held'.` and further means alongthebeam path for absorbing those electronssofV the beam which derive.. energy from the eld.

5. A thermionic tube structure comprising a hollow resonator having opposed apertured walls, an.y entrance:= grid occupying an:4 aperture infone of said walls, a reflecting: electrodefoocupying'an aperture .in theopposed walk-@means opposite-said entrance. grid for` projectinga-beam of electrons through` said. entrance gr-idi `for exciting electromagnetic waveswithin said-resonator, andlfllter meanscoupled. to .saidreectingf elect-roda Vfor preventing escape of.` said-V electromagnetic =waves from saidfresonator. past-said reflecting electrode.

6.4 A` thermionicmtubef structureY comprising'- a hollow resonator yl'lavingbpposedl aperturediwalls, an.entranceigridpcupying an aperture ineoneof said walls,. a reecting.y electroderoccupyingr an aperturein'the. opposed wall,I means opp'ositesai'd entrancev grid..for..projecting l a'f beam of electrons through said entrancegrid-for exciting electromagnetic waves within said resonator, a secondary electron emissive coating'on said reflecting electrode,l and -filter meansv coupled to'said reecting electrode for preventing escape of said electromagnetic waves from said resonator past said reflecting electrode.

7. Ultra high frequency apparatus comprising means for producing an electron stream, a hollow conducting resonator along the path of said stream and adapted to contain an oscillating electromagnetic field, means including a reflector electrode positioned within said resonator for refleeting said stream back outwardly of said resonator, and filter means coupled to said reflector electrode and including said reflector electrode as an element thereof for preventing fundetired radiation of the electromagnetic energy from said resonator by way of said reflector electrode.

8. High frequency apparatus as in claim 3 wherein said means for abstracting high frequencyenergy4 froml said 'resonator includes at least-a portion of saidV filter; meansi 9. Ultra'highffrequency'apparatus Comprising mean's'for'producing an electron stream', a cavity resonator having `an electron-permeable wall along the path of said stream, a` reflectory electrode alongthe path 'of` said stream and insulated from said resonator, filter means coupled tof and including said-reiectoras an element" thereof for preventing' undesired radiation of'electromagnetic energy from said resonator by way of said electrode, and means coupled' to'said'y reflector electrode and said resonator for applying a potential" difference therebetween.' to cause'said' beam tobe reflected-back'through said-resonator toexciteelectromagnetic oscillations therewithin, said last-named means' including said' lter'as a portion thereof.

10. High frequency'apparatus` as' in claim 9, further includingmeanscoupled to saiclresonator for abstracting'high frequency'energy from said resonatorby'way ofLat least a portiorrof sai'dlter means.

11. High frequency apparatus comprising an axiallysymmetrical cavity' resonatorhaving an entrance meanson the axisithereof'throughwhich anV electron beamV is adapted` to pass; said entrance means comprisingan inwardly1curvedv grid, a reflector'4 electrode forminga portion ofthe wall of said resonator/"and insulated'therefr'om, said electrode having a concave surfacev oppostepand substantially vequi-.distant from corresponding portions of' saidgridmeans" aligned with said grid and" reflector for projecting.V van electron beam alongi said' axis` through said grid toward said reflector, .and means coupled to said reector and' resonator'forl applyingI a potential. difference therebetweenA to cause' saldi beam to b'e reflected back through said grid;

12. High frequency Iapparatus comprising. a hollow resonatoradapted to contain anoscill'ating electromagneticY field and having an., apertured wall anon-planar entrance, grid ,occupying an aperture in said wall and. bowedv inwardly into said resonator, a correspondingly concav'ely formed reflecting electrode, .means for` projectingv a beam of electrons through; said grid toward said electrode for' exciting(electromagnetic.Joscillations within said" resonator,f whereby' the. non-1 planar construction of saidgri'd and reflector focuses the electron beam passing throughsaid resonator andltermeans including saidelectrode as an element 4thereof. for, preventing undesired radiation from saidresonator.

13. A generator of' electromagneticwaves as in claim 41 wherein saidlast-namedmeansincludes' a collector electrode. in the. path. of` said beam for collecting those electrons which pass through said second permeable Wall.

14. High frequency apparatus comprising a hollow resonator adapted to contain an oscillating electromagnetic eld and having an apertured wall, means aligned with said wall for projecting electrons into said resonator through said wall, means including an electron-permeable reflecting electrode in the path of said electrons for reflecting at least a portion of said electrons to excite oscillations within said resonator, and means along said path for absorbing those electrons which 'derive energy from the field within said resonator.

15. High frequency `apparatus comprising a cavity resonator having an entrance means through which an electron beam is adapted to pass, means aligned with said entrance means for projecting an electron beam therethrough, a reflector electrode in the path of said beam and having an insulated electron-permeable grid forming a portion of the wall of said resonator and having means for collecting electrons passing through said grid, and means coupled to said reilector and resonator for applying a potential `difference therebetween to cause at least a portion of said beam to be reversed in direction.

16. A generator of electromagnetic waves comprising means for producing a linear electron stream along a predetermined axis, a hollow electromagnetic resonator adapted to contain an oscillating electric field therein and mounted along the path of said stream, said resonator being symmetrical about said axis and having an aperture surrounding said stream path, electronu permeable means mounted in said resonator aperture in said stream path and bowed inwards of said resonator, and means including an electrode forming part of the wall of said resonator but insulated from said resonator and in the path of said stream for opposing the flow of said electrons, said electrode having a, concave surface for coacting with said permeable means to focus said stream, whereby the field within said resonator effects velocity modulation ofsaid stream and said opposing meanseffects density moduation of said velocity-modulated stream, said density modulated stream thereby supplying energy to maintain said field. Y

17. Intra-high-frequency apparatus comprising means for producing an electron stream, a cavity resonator having an electron-permeable wall along the path of said stream, a reflector electrode along said path and insulated from said resonator, and filter means coupled to and including said reflector electrode as an element thereof for preventing undesired radiation of energy from said resonator.

18. High frequency apparatus comprising means for producing a stream of electrons along a. predetermined axis, a :cavityresonator axially symmetrical about said axisand having an entrance means along the path of said stream, said entrance means comprising a non-planar electron-permeable grid, a reilector electrode forming a, portion of said resonator and insulated therefrom and having a non-planar portion in said path of a shape corresponding to said grid, whereby said grid and reflector are maintained at a substantially equal separation over their entire facing surfaces.

19. High frequency oscillation generating apparatus comprising means for producing a linear stream of electrons along a predetermined axis, a cavity resonator axially symmetrical about said axis and having a non-planar electron-permeable 12 grid in the path of said stream, and a reflector electrode along said path forming a portion of the Wall of said resonator and having a corresponding non-planar portion maintained at substantially equal distances from corresponding portions of said grid.

20. High frequency oscillation generating apparatus comprising means for producing a linear stream of electrons along a predetermined axis, a cavity resonator axially symmetrical about said axis and having a non-planar electronpermeable entrance grid in the path of said stream, and a reflector electrode forming a wall of said resonator opposite said grid and along said path.

21. A thermionic tube structure comprising a hollow resonator having opposed apertured walls, an entrance grid occupying an aperture in one of said Walls, a reflecting electrode occupying an aperture in the opposed wall, means opposite said entrance grid for projecting a beam of electrons through said entrance grid, said reflecting electrode having a secondary electron emissive coating thereon on the surface facing the said entrance grid, whereby a portion of the electrons of said beam may be caused to strike said coating to produce secondary electrons for assisting in the production of electro-magnetic oscillations within said resonator.

DONALD L. SNOW. EDWARD L. GINZTON. DONALD R. HAMILTON.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Nurnber Name Date 2,406,850 PierceY Sept. 3, 1946 2,128,235 Dallenbach Aug. 30, 1938 2,141,080 Y Dallenbach Dec. 20, 1938 2,278,210 Morton Mar. 31, 1942 2,200,023 Dallenbach May 7, 1940 1,844,319 Hatt Feb. 9, 1932 1,877,872 Hollmann Sept. 20, 1932 2,314,794 Linder Mar. 23, 1943 2,170,219 Seiler Aug. 22, 1939 2,128,232 Dallenbach Aug. 30, 1938 2,157,952 Dallenbach May 9, 1939 2,190,511 Cage Feb. 13, 1940 2,167,201 Dallenbach July 25, 1939 2,128,236 Dallenbach Aug. 30, 1938 2,250,511 Varian et al July 29, 1941 FOREIGN PATENTS Number Country Date 665,619 Germany Sept. 29, 1938 

